Okay, so I made this modification some time back and decided that it wasn't
worth the effort considering that my Dob was stored outside in a shed for
most of its life. However, having recently moved into a new home (with no
shed available), and since the observatory isn't even close to being
started, the scopes are now stored in the garage. The temperature
differential is hence greater than it used to be (garage is
attached/insulated), and the results are now more obvious.

The fan modification is definitely worth the effort for scopes that have to
transition even a mere 5 degrees F. Previously, I also had thought that fan
vibration was a problem, but I no longer believe that to be the case. What
might be the problem is the velocity at which the air flow is sweeping the
mirror as I am still running at the full fan speed at 12VDC. Velocity of air
flow is quantifiably visible in the out of focus image during star test
collimation. I think I was interpreting that as vibration in earlier tests
(and may even have stated this theory at that time). (I really need to get
speed control. Can someone remind me how to do this with a 12VDC fan? (And
don't say 9VDC battery)).

I think the main difference with the fan is the controlled "sweeping" of air
across the mirror, in lieu of the turbulence of natural convection. The
sweeping of air is of even greater benefit than simply cooling the glass, as
it gives an immediate result. I was very nicely surprised at last night's
results.

Adler rocks!

--
-Stephen Paul